Randomly, videos within an eLearning won't play

I've been contacted by a few of my learners that the videos within the course don't play properly or at all.  I've tested and tested and can't reproduce this (it works perfectly for me).  It's only affected a few learners.  What could be causing this issue?  There is no connection between the learners (different browsers, different devices, hardwired or wifi).  Could this be a problem with my course or something external?

The user is able to click on the button to open either a layer or new slide and as the video is triggered to play automatically.  The layer and/or new slide opens but the video won't play, even when they try using the "video controls".